With rapid advance of manufacturing process, any advance process control (APC) designed for improving production yield must be able to perform a real-time in-situ analysis relating to parameter control while feeding back the analysis results to the APC for parameter adjustment. In the process of current semiconductor fabrication, feature size had been scaled down to 65 nm which is already smaller than optical diffraction limit. As it is noted that a projected feature size of 65 nm is beyond the resolution limit for any conventional microscope, the conventional microscopic imaging can no longer meet with the requirement of APC. Although both the scanning electron microscope (SEM) and atom force microscope (AFM) can provide sufficient resolution, they are not preferred since the electron beam of SEM will cause electric charge to accumulate on the specimen surface and thus cause the specimen to damage, not to mention that SEM can only inspect the surface structure of a specimen and lack the ability for internal inspection; and the AFM, being also incapable of internal inspection, is notoriously slow for most industrial inspection applications that it is not suitable to be used in any in-situ inspection.
It is known that the scatterfield microscopy is substantially a non-contact optical microscopy that not only it is capable of achieving a resolution beyond the diffraction limit, but also it can inspect a specimen of multi-layered structure by regression calculation in a speed fast enough for any in-situ inspection. Thus, scatterfield microscopy is becoming commonplace in many new-generation advance process control for semiconductor fabrication and inspection, and, there are already some semiconductor manufacturers who had applied scatterfield microscopy in their in-situ inspection processes.
There are already many studies relating to the application of scatterfield microscopy. One of which is a microscopic system disclosed in “Scatterfield microscopy using back focal plane imaging with an engineered illumination field”, Proc. of SPIE, vol. 6152, 61520J(2006), by H. J. Patrick, R. Attota, B. m. Barnes, et al. As shown in FIG. 1, the microscopic system 1 adopts a structure of bright-field reflection microscope with a mask 10, through with an image can be formed on the back focal plane of a objective lens 12 over the transmission of a relay unit 11. As the mask 11 is controlled to move by a control unit in a precise manner for changing the illumination angle of an incident beam 13 upon a sample accordingly, the imaging unit 14 is able to record scattering light of various angles.
Another such study is an inspection device disclosed in U.S. Pat. No. 7,061,623 B2, entitled “Interferometric back focal plane scatterometry with Koehler illumination”. In an interference microscope used in the aforesaid U.S. patent, the positioning of a sample of that of a reference surface is controlled to move in a precise manner by a control unit, only the portion of an illumination light of specific characteristics is allowed to project on the sample while the other portion of the illumination light are blocked by destructive interference. Basically, the aforesaid patent use an interference spectroscopy for selecting incident beams of certain illumination angles to shine on the sample while using the same to register the reflection thereof.